Conventional sheet feeders which feed sheets from the top of a stack frequently skew the sheet being fed, requiring the use of a separate sheet jogging or straightening device to straighten the sheet before it is fed to the apparatus with which the sheet feeder is being used. The use of a sheet jogging or other conventional sheet straightening devices adds extra expense and requires the sheet to travel a sufficient distance along the feed path so that it can be straightened, thus resulting in a sheet feeder having a longer feed path and a greater size than otherwise necessary. This is particularly a problem when the sheets are fed at high speeds using a driven feed wheel. Such a feed wheel often does not make even contact with the top sheet to be conveyed along the entire longitudinal length of the feed wheel, and a skewed feed results.
Other problems are encountered when attempting to feed sheets selectively from two stacks positioned on spaced-apart trays. One problem is in inserting one or more sheets from one stack between sheets from the other stack upon command from the apparatus with which the sheet feeder is used in a manner which provides a rapid, smooth and continuous flow of sheets from both stacks onto a single feed path. Another problem is in feeding the sheets to the apparatus with which the sheet feeder is used within a short period after a sheet request signal is given. This is necessary so that the operating speed of the apparatus is not unduly slowed by the sheet feeder and so that the apparatus does not register an error because a sheet has taken too long to reach the apparatus after the sheet request signal has been given.
Another problem often encountered with sheet feeders involves the feed wheel feeding more than one sheet at a time from a stack. In the past, such an error condition has been successfully indicated through use of a doubles detector such as described in U.S. patent application Ser. No. 480,121, filed on Mar. 25, 1983, now abanoned. Such a doubles detector, however, must be accurately calibrated by passage of single and double sheets between a pair of rollers to preset the roller spacing. The spacing is set such that a single sheet will freely pass between the rollers, but double sheets will cause the rollers to move apart and trigger a sensor. In the past, the calibration has been done by hand, and achieving an accurate setting has been time-consuming and subject to operator error.
It will therefore be appreciated that there has been a significant need for a sheet feeder designed to overcome these problems. The present invention fulfills this need and further provides other related advantages.